Rotate view around vertical Y-axis
By default, blender is set up with turntable navigation style, which means the Z is held vertical through your orbit manipulation. There is another style of navigation which used to be common in 3D, called trackball. This lets you orbit without the restriction of Z-up, but it also makes it easy to lose your sense of space in the scene. You can change this setting in the user preferences, under the input section. The left column contains those settings.

In 2.8+ &quot;trackball&quot; option is now in the &quot;navigation&quot; tab of the preferences: